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   Home   Help Search  
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Author Topic: Hallicrafters HT33A  (Read 4184 times)
AE6PR
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Posts: 14




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« on: April 02, 2016, 04:53:56 PM »

I am restoring this amp. The original filament transformer is missing. I have solid state rectifiers and a small bias transformer. I bought it with a 6.3 volt 8 amp filament transformer installed. Two things. 1. Can the PL172 handle 6.3(it is rated for 6.0 volts.) 2. the unloaded voltage measures 7.6 volts. I do not want to ruin this rare tube by overvolting the filament. Any thoughts? Thanks
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K6AER
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« Reply #1 on: April 02, 2016, 07:14:48 PM »

I would bring up the tube filament to  6 volts with a variable power supply and see what the tube current is. Then add the appropriate dropping resistor to the transformer output so the tube sees 6 volts.
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G3RZP
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Posts: 1273




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« Reply #2 on: April 03, 2016, 01:49:55 AM »

If you can't find an appropriate low value resistor for putting in series with the transformer secondary, you can use a higher value in series with the primary, which might be easier to find.
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AE6PR
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« Reply #3 on: April 03, 2016, 10:19:01 AM »

One observation. When I brought the plate transformer on line with a variac it really was noisy at about 40 volts line input. I tried it a few times with the same results. I checked every part of the circuit for shorts and measured the choke resistance. Everything was in spec. I went for broke and increased the voltage past 40 volts and the transformer settled down, quite as a mouse and full output at 115 volts. I let it cook for a few hours-no problems. Must have been a weird resonance between the choke and transformer at low input voltage. Anyone hear of this? It is a 50 year old amp after all.
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KD6VXI
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Posts: 177




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« Reply #4 on: April 03, 2016, 11:34:21 AM »

That would be normal for a choke input supply.

--Shane
KD6VXI
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WA1RNE
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Posts: 1010




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« Reply #5 on: April 03, 2016, 12:03:38 PM »

 According to the OEM datasheet, the recommendation for filament voltage is:

 -  to maintain it as close as possible to 6.0 volts

 -  voltage variations up to 10% are permissible, but more than 5% will shorten tube life

https://frank.pocnet.net/sheets/201/8/8295.pdf

 An increase of +5% is 6.3 volts. Depending on the transformers load regulation characteristics and the transformer wiring, it's possible you may be within the safe 6-6.3 volt range.

 You can either load the transformer with a 0.73 ohm load (tube is rated 6 v / 8.2 amperes) or hook it up outboard of the amplifier and bring it up with a variac while monitoring the output voltage AT THE TUBE SOCKET and checking the line voltage to the transformer to insure it's what your line voltage is at your location.

The only thing this does not take into account is a high line condition. If it's typical for your line voltage to periodically increase +5% for short periods of time you will still be within the 10% rating, but if it runs higher than that for long periods you may want to reduce the voltage to 6.0 volts.

 At 6.3 volts, you will need a resistor of 0.038 ohms to get the voltage down to 6 volts at the tube. You can use (3) 0.1 ohm resistors in parallel which will give you a 0.033 ohm resistor which will work fine. The 0.1 ohm resistors should be metal film and will be dissipating 0.75 watt each so use 2 watt rated types and you will have plenty of margin.

...WA1RNE
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AE6PR
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« Reply #6 on: April 03, 2016, 12:28:21 PM »

Thanks all for the info. I will try it.
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W7APM
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Posts: 60




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« Reply #7 on: April 05, 2016, 03:08:43 PM »

I have experience with the PL-172 tubes and some have gotten gassy after many years. I would burn it it for a day or so before I put HV on it...
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AE6PR
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Posts: 14




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« Reply #8 on: April 05, 2016, 07:56:54 PM »

Well, I put the spurs to it today. Turned on the HV and nothing blew up! I did cook the tube for a few days on filament alone. I will run it on cut off for a few hours to condition the tube a bit more. Thanks all.
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WB2WIK
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Posts: 21836




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« Reply #9 on: April 06, 2016, 10:37:44 AM »

Health tip:

Disconnect amp from everything and carry it around a few hundred feet.

Bonus points if you carry it up a long staircase. Wink
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KM1H
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Posts: 5259




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« Reply #10 on: April 06, 2016, 05:43:33 PM »

Next step is to run it with idle current only and let that cook for a few hours if it doesnt kick off. This will regetter internal gas but will do nothing for leaky seals which the glass base PL-172/8295 is known for. The 8295A has a ceramic base and behaves well.

Carl
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AE6PR
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Posts: 14




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« Reply #11 on: April 19, 2016, 08:35:43 PM »

One more question. I mounted the 4 10k 50 watt resistors vertically instead of horizontally. Originally they were next to the oil filled cap but since it has ss rectifiers instead of mercury tubes now they now are vertical. These resistors run at the max ( I calculated 47 watts dissipation) should I fan cool them? This configuration is per the schematic. I cannot envision these heaters mounted so close to the cap but that is how the factory did it. Anyone with experience with this amp remember the heat ?
Thanks
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N3AJB
Member

Posts: 60




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« Reply #12 on: April 20, 2016, 05:59:26 AM »

Hi,

I rebuilt several HT 33bs decades ago.  With regard to the big screen dropping resistors next to the filter cap, place a ceramic tile of suitable size between the capacitor and resistor to act as a heat shield. 

The PL 172 was of very poor production quality and many were short lived.  In addition, it uses a glass base which was another weakness.  Turn the tube over and you will see a small pin in the center of the base.  A metal tab (about .25" square) should be attached to dissipate heat from the glass base to prevent cracking.  The tab was easily lost and almost never attached to the tube.  Most people were not even aware of it's function.  It's easy to make the tab and force-fit it to the metal pin.  Make sure it's small enough so it does not touch other parts of the tube socket. 

Heat from the filament will not de-gas a glass envelope tube (but it will for a ceramic tube).  To de-gas a glass tube (the process is called gettering), a low value of HV is applied to the tube  (800 - 900V) and the amp is tuned out of resonance with a very low amount of plate current - 5 -10ma - flowing.     See W8JI's web site for more details.  Finally, the Eimac 8295A ceramic tube is a direct substitute for the PL72.  If you can find one one at a reasonable price use it in place of the PL172. Be sure to run the filaments for several hours to de-gas it.

JON N3ABJ
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